1. Field of the Invention
The invention relates to resistor structures for high current semiconductor devices and processes for forming them, and particularly to the forming of precisely doped polysilicon over an emitter region to form a vertical ballast resistor.
2. Description of Prior Art
Ballast resistors are commonly used in high current semiconductor devices to assist in the distribution of current flow among a set of emitter fingers. The use of such ballast resistors assists in insuring that each emitter finger carries an appropriate share of the total current flow. By balancing the current flow, the maximum current flowing through the entire device may be increased.
Typically, as will be described in more detail below, such ballast resistors are configured to ballast the current load only among the emitter fingers. There is no ballasting of the current flow across the length of each emitter finger, and as a result different portions of each emitter finger are subjected to different current flows. As a result of this "one dimensional" ballasting, prior art high current semiconductor devices have an undesirable thermal profile caused by the varying amounts of current flow through portions of each emitter finger. The resulting hot spots in such a prior art device limit the total current flow possible through it due to the possible thermal runaway of the device at the hot spots, and corresponding destruction of the device.
Accordingly, it is the object of the herein disclosed invention to provide a structure and associated fabrication processes for a high current semiconductor device in which the current flow through portions of each emitter finger is ballasted to reduce the likelihood of thermal runaway. An additional object of the invention is to provide for a high current semiconductor device having improved maximum current flow than that otherwise obtainable. Another object of the invention is to provide a process for precision forming of a polysilicon based resistor over a site in a semiconductor device. Another object of the invention is to provide for a two dimensional ballasting of an emitter structure comprising plural emitter fingers in which ballasting is provided across the length of each emitter finger and among the emitter fingers.